I am wrapping a wire around the cable that goes to the spark plug, so that a pulse is induced in this wire each time the engine sparks.
That was perfectly clear. That pulse can be hundreds, to thousands of volts, positive and negative, into a high impedance load.
I didn't think noise would be an issue.
I'm quite certain that it is, having seen quite a bit of circuitry destroyed by electrical noise in automotive electrical systems. And, you are here because you have a problem!
The entire circuit needs to be in a grounded metal box, and the signal wire needs to be shielded, with the shield grounded.
Good luck with your project.